The present invention relates to radial and bias/belted tires having tread ends joined by a tread splice adhesive composition and more particularly to a universal cement suitable for both natural and synthetic rubber-based tire compounds.
Natural rubber is used in tire adhesive compositions because it provides good tack to both natural and synthetic rubber compounds, but the use of substantial amounts of natural rubber (e.g., 20 phr or more) in tire tread and carcass adhesives applied to synthetic rubber tire compounds causes poor cured adhesion.
Tack may be defined as the ability of two uncured rubber materials to resist separation after bringing their surfaces into contact for a short time under a light pressure.
In synthetic rubber tires, synthetic rubber adhesive compositions are employed which contain phenolic tackifying resins to improve tack and which provide good cured adhesion. Various compositions have been proposed. For example, U.S. Pat. No. 3,342,238 discloses tread and carcass adhesives based on styrene-butadiene (SBR) rubber containing 30 or more parts by weight ("wt") of phenolic tackifying resins per hundred parts rubber (phr) to be used for adhering conventional SBR tread and carcass compounds. Although the tack of SBR rubber is notoriously poor, building tack can be improved by use of tackifying resins such as Koresin or certain alkyl-phenol aldehyde resins.
Attempts have been made to improve tack by increasing the amount of the tackifying resins as in U.S. Pat. No. 3,514,423 which discloses tread splice cements based on high-Mooney SBR rubber containing 65 or more phr of Koresin and optionally up to 30 phr of a cis-polybutadiene. However, tread end cements of this type do not have the properties required for modern radial or bias/belted tires, particularly at the critical tread splice.
Modern tires employ a plurality of circumferential belts between the tire carcass and tread with wires or textile cords embedded in a natural rubber-based rubber compound and employ a two-layer (dual) extruded tread comprising an outer cap portion formed of an abrasion-resistant synthetic rubber composition and an undertread portion formed of a natural rubber-based compound better suited for adhesion to the outermost belt. Serious problems have heretofore been encountered in such tires in the region of the tread splice because of poor tack and poor cured adhesion.
Prior to the present invention, no tread splice adhesive composition was available which provided good tack and good dynamic cured adhesion properties when applied to both natural rubber and synthetic rubber tread compounds. The synthetic rubber-based adhesive performed poorly with natural rubber-based compounds because of poor tack, and the natural rubber-based adhesives performed poorly with synthetic rubber-based compounds because of poor cured adhesion. This problem has existed for many years, and no solution was found. The rubber industry has for many years needed an effective synthetic rubber-based tire adhesive composition with good tack to a natural rubber compound but none was found.
One of the most serious problems encountered in the manufacture of synthetic rubber tires is the poor tack provided by synthetic tire adhesive compositions, such as commercial SBR-based tire cement compositions, particularly after aging for a substantial period of time, such as 20 hours (hr) or more. Tack retention is particularly important in a tread splice adhesive because of the critical importance of the splice. After aging the tack provided by a typical SBR-based tread-splice cement is poor and frequently inadequate for tire building when the cement is applied to an SBR-based tread cap compound having a low oil content and/or to an NR-based undertread compound as in typical radial tires having an extruded dual-layer tread.
In building such radial tires, an SBR-based tread cement is employed to splice the tapered cut ends of the dual-layer tread, and an NR-based tread cement is employed to adhere the outermost circumferential belt to the NR-based undertread. The SBR-based cement is normally applied by a brush or sprayer in a fast haphazard manner to the tread ends during tire building and usually slops over a portion of the underlying circumferential belts. This is highly undesirable because of the adverse effect on tack and is a source of defects in the tire adjacent to the tread splice.
The frequency of adhesive failures in modern tires has been reduced by improvements in rubber compounding and in tire building machines and building procedures, but the critical need for better tire adhesives has long been recognized particularly because of the adhesion problems associated with modern radial and bias/belted tires.
Blends of major amounts of SBR rubber and minor amounts of BR rubber are used extensively in tire tread compounds. In some cases, minor amounts of BR rubber are blended with natural rubber in truck tire treads. However, polybutadiene elastomers are considered to have poor processing characteristics and poor tack and have found little use in modern commercial tire adhesive compositions. Heretofore it was not recognized that cis-polybutadiene elastomers could be important in modern tire adhesive compositions or that major amounts of such elastomers were practical or desirable in such compositions.